1. Field of the Invention
The present invention relates to electroluminescent structures and, more particularly, to an electroluminescent structure having increased brightness and resolution for electroluminescent displays.
2. Description of the Related Art
Electroluminescent (EL) displays produce light when an alternating current (AC) voltage is applied across a phosphor film sandwiched between a pair of electrodes. Referring to FIG. 1, electroluminescent light originates from metal activator atoms that are introduced into a phosphor film 12 and excited by energetic electrons as they move across the semiconducting phosphor film 12. Since the phosphors employed have large band gaps, visible radiation produced (indicated by arrows) passes through film 12 without absorption and out of the stack through a transparent electrode 14.
The typical EL film stack contains two dielectric layers 16 and 18, one at each electrode interface, i.e. one for transparent electrode 14 and one for electrodes 20. These dielectric layers limit the current through the structure and prevent a catastrophic breakdown should a phosphor imperfection produce a conductive path through film 12. Dielectric layers 16 and 18 also store charge, increase the internal electric field and reduce the effective turn-on voltage of the phosphor. Thin 500 to 1000 A films with high dielectric constants are often used to enhance the effect and increase the luminous efficiency of EL displays.
Compact high-resolution displays have been produced with on-chip scanning and pixel control circuitry. In these "active matrix" displays or active matrix electroluminescent (AMEL) displays, the necessary dielectric, phosphor and transparent electrode layers are deposited and defined as a single rectangle over the entire pixel array. Referring again to FIG. 1, individual pixel electrodes 20 are controlled by switching a transistor 22 which blocks the AC phosphor excitation voltage 24 when "off" and allows passage current through the phosphor when "on". Pixel electrodes 20 are positioned directly over the controlling transistors, to maximize resolution.
While this architecture addresses the information content and size requirements of small displays, the structure limits the brightness and resolution achievable. Pixel electrodes translate the underlying topology of the active matrix array and present an irregular surface that does not efficiently reflect phosphor radiation toward a viewer. Light emitted from one pixel can migrate from the electrode to neighboring pixels through lateral emission and internal reflection in the phosphor film stack to degrade resolution and color spectral purity as shown in FIG. 1. Pixel electrode structures that maximize the phosphor emission and transmission efficiency and minimize lateral light diffusion are needed to satisfy the requirements for high brightness, color and high resolution display products.
These and other factors contribute to difficulties associated with ambient light conditions. Implementation of emissive displays in high ambient light situations is currently restricted by the brightness level of the displays. Application of the displays in this type of environment requires significant improvements in existing materials and structures. Significantly brighter emissive displays, for example having a luminance of 1000 ft-Lamberts are needed to provide adequate display images to a viewer in high ambient light conditions with conventional optical systems.
Miniature active matrix displays are typically fabricated with integrated circuit processes on pilot or production lines using structures and materials that are in general use in the semiconductor industry. The circuits and devices employed resemble closely the elements that are used in conventional semi-conductor CMOS chips. Device interconnections are usually constructed from familiar polysilicon, silicide and aluminum films. These materials have become increasingly inadequate for the stringent requirements needed for active matrix displays.
In active matrix displays (or AMELs), for example, tungsten or aluminum metal alloys with refractory metal capping layers have been used for circuit interconnection and electrode fabrication. These materials provide the chemical and thermal stability necessary for compatibility with a subsequent phosphor deposition and activation steps (for an electroluminescent layer). Further the tungsten is used for its electrical conductivity, etchability and film coating conformality, all of which contribute to the reliability needed for high yield circuit interconnection.
Therefore, a need exists for highly reflective electrode structures in electroluminescent displays to maximize luminance and broaden useful applications of the displays. A further need exists for the high reflective electrode structures to retain their characteristics throughout the fabrication process and operational life.